Method of getting rid of malodorous air and water pollutants from alkaline pulp cooking



July 1970 E. A. s. LINDBERG 3,520,772

METHOD OF GETTING RID OF MALODOROUS AIR AND WATER POLLUIANIS FROM ALKALINE PULP COOKING Filed April 25, 1966 INVENTOR.

United States Patent 3,520,772 METHOD OF GETTING RID OF MALODOROUS AIR AND WATER POLLUTANTS FROM ALKA- LINE PULP COOKING Erik Axel Sigvard Lindberg, Skoghall, Sweden, assignor to Uddeholms Aktiebolag, Uddeholm, Sweden Filed Apr. 25, 1966, Ser. No. 545,017 Claims priority, application Sweden, Nov. 15, 1965, 14,714/ 65 Int. Cl. D21c 11/08 US. Cl. 162-51 1 Claim ABSTRACT OF THE DISCLOSURE In an alkaline pulping process, especially the manu facture of so-called sulfate pulp, the noxious and malodorous gases and vapors leaving the digesters in the cooking of the wood chips, without passing through a condenser, are brought, together with steam from the digesters directly to a furnace, where the gases are burned or rendered innocuous by thermal decomposition. In the process here disclosed, the pollutant gases leave the digester together with steam and are passed directly to a furnace without passing through a condenser. The furnace where the combustion takes place is preferably associated with a conventional boiler, e.g., a soda recovery boiler, or a continuous lime kiln. The gases flow from the digester to the furnace without condensation of noxious constituents which when condensed would only be a different kind of nuisance; but they may pass through a superheater in order to prevent such condensation during such passage. Thus, not only is the atmosphere in the vicinity of the pulp mill protected from pollution by the discharge of noxious gases, but the streams and lakes and low places which might be used as settling ponds are also protected against pollution. According to the disclosure, the waste gases consisting of the pollutants mixed with steam from the cooking process and oxygen for the burning are proportioned so that the oxygen concentration is below that of an explosive mixture, so that the combustion is safe. The heat content of the gases can be recovered by use of a flue gas scrubber.

The object of the present invention is to get rid of malodorous air and water pollutions by combustion of gases from the cooking in a continuous digester of alkaline pulp, especially sulphate pulp.

At the manufacture of pulp by sulphate cooking malodorous mercaptans and organic sulphides are formed. They are volatile and they leave together with aqueous vapour and uncondensable gases partly during the cooking (relief) and partly at the blowing of the digester (blow). During the last decade several methods have been Worked out to remedy the sanitary inconvenience caused by these gases. A review of the methods used in Europe has been published by Adams, Tappi 48, 5, pages 83A-87A, 196-5. Significant for all these methods is that the gases are cooled in a condenser. The main part of the water content and a part of the malodorous compounds are condensed, while the rest of these compounds and the uncondensable gases are destroyed chemically either by oxidation or by combustion.

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Such combustion takes place in a soda recovery boiler, in a lime kiln or in a furnace especially built for this purpose. The methods hitherto proposed and/or realized to get rid of the odour all have in common that the gases from the sulphate digester, both relief and blow gases, are first partially condensed, whereafter the part of the malodorous compounds, which are condensed, either leaves the mill as a water pollution or requires further treatments to be destroyed. For the oxidation spent bleaching liquors can be used. However, they are often not suflicient, for which reason extra chemicals, usually chlorine, are required for the oxidation. Even though the main part of the injurious compounds can be oxidized, it has been found that it is diflicult to destroy the malodorous compounds in a chemical way.

From the odour point of view, the combustion of the gases is a more eflective way, which, however, is not free from problems. The gas mixture obtained is often explosive and hence dangerous to handle. Therefore, one has tried to keep the composition outside the explosion limits by keeping the admixture of air either above or below the risk range. In addition, it has been tried, as a protective measure, to minimize such risk by keeping the gas velocity higher than the rate of propagation of explosion. Also, bursting discs have been used to reduce the damage from an explosion; flame arresters have been used in front of the combustion chamber as well.

An object of the present invention is to make it possible in one single operation, to avoid pollution by the process, both in the air and in water, where the invention is being carried out. According to the invention, the pollutant gases to be destroyed which leave the digester, together with steam, are brought directly to a furnace without passing through a condenser. The furnace where the combustion takes place is preferably associated with a conventional boiler, e.g., a soda recovery boiler, or a continuous lime kiln. Sometimes the gases are passed through a superheater between the digester and the furnace in order to prevent the water from being condensed. By the invention the advantage will be gained that all malodorous compounds are effectively destroyed without any stream pollution. On account of the small content of oxygen and high content of aqueous vapour the digester gas mixture cannot be explosive under any circumstances and the combustion will be safe. If the combustion takes place in a plant provided with a flue gas scrubber, the heat content of the steam in the gases will also be recovered. Compared with other systems the invention has the economical advantage of relatively low investment costs.

The invention is applied with advantage in continuous cooking. The attached drawing shows schematically one construction form of the invention. Into a steaming vessel 2 belonging to a digester 1, chips are brought through a sluice valve and steamed by direct vapour together with vapour brought back from the digester circulation through a cyclone 3. Air and turpentine from the chips and malodorous compounds which have been collected from the digester are drawn off through a pipe 4. Wash liquor is introduced in the bottom part of the digester and black liquor is taken out and brought to a cyclone 5 where it is freed from steam and gases which are conveyed through a pipe 6. The pipes 4 and 6 are connected and the gases from both sources are led through a superheater 7 and from there to a soda recovery boiler where they are burnt.

Of course, the gases can be brought separately to a suitthrough a condenser, whereby to prevent liquid particles able furnace with or without superheating. from entering said furnace.

What I claim is:

1. An alkaline sulfate pulping process, which comprises References Clted rendering innocuous for pollution of air or water the gases UNITED STATES PATENTS emitted during cooking in a continuous digester, by con- 5 2,830,869 4/1958 Limerick 16251 ducting said gases mixed with steam in sufficient concen- 3,311,531 3/ 1967 Feischl 162-51 tration to prevent the gas mixture from being explosive, superheating said gases in a heat exchanger, passing the HOWARD CAINE Primary Exammer superheated gases directly to a combustion furnace from 10 US. Cl. X.R.

their source, and burning them therein, without passing 2l55 

